Liquefied natural gas (LNG, hereinafter, called ‘LNG’ for convenience's sake in description) is obtained by liquefying natural gas extracted from a gas field. LNG is distinguished from liquefied petroleum gas (LPG) in that the chief ingredient of LNG is methane. The pressure of LNG is reduced to 1/600 when the LNG is liquefied by pressure applied, but because the boiling point of methane is at low temperature of −162° C., methane is cooled or compressed, and then, is transferred to a tank which as specially insulated. LNG is mainly used as city gas because it is a colorless and transparent liquid which has little pollutants and has a high heating value.
In the meantime, a ship which is manufactured to transport LNG is called a liquefied natural gas carrier (LNGC, hereinafter called ‘LNG carrier’ for convenience's sake in description). Such an LNG carrier includes an insulated cargo tank (hereinafter called ‘LNG tank’ for convenience's sake in description) which can store LNG.
As shown in FIG. 1, the LNG tank of the LNG carrier includes: an LNG tank outer wall 50 which is made of metal strong to low temperature and disposed inside a ship outer wall 15 made of metal; a second insulation area 70 having an LNG tank outer wall 50 and a second adiabatic membrane 30 therein; a first insulation area 60 having the second adiabatic membrane 30 and a first adiabatic membrane 20; and thermal insulators 40 respectively disposed in the insulation areas.
Because the first adiabatic membrane 20 forms an inside space of the LNG tank and directly comes into contact with LNG of an extremely low temperature, if the first adiabatic membrane 20 leaks, LNG penetrates into the first insulation area 60 so as to put the LNG carrier in danger of explosion. Here, the adiabatic membrane is made of stainless steel and invar which is an alloy of which main components are iron and nickel and thermal expansivity is very low. The adiabatic membrane is fabricated by assembling and welding a thin sheet which is 0.7 mm to 3 mm in thickness into a predetermined size. The thermal insulators 40 are fabricated by glass fiber, perlite and urethane foam formed into the predetermined size and are respectively assembled between the LNG tank outer wall 50 and the second adiabatic membrane 30 and between the second adiabatic membrane 30 and the first adiabatic membrane 20 in order to insulate therebetween.
Because the LNG tank of the LNG carrier stores and carries ultralow LNG of 162 degrees below zero which is compressed and liquefied at high pressure, the LNG tank receives a structural stress such as continuous compression and expansion according to changes in pressure due to loading and unloading of LNG. Moreover, the LNG carrier carries LNG while sailing in rough seas and uses six degree-of-freedom motions (roll, pitch, yaw, and so on) while carrying LNG. Therefore, sloshing as shown in FIG. 2 is caused by liquid slosh inside the LNG tack, and it continuously applies shock to the structure, namely, the adiabatic membrane of the LNG tank so that fatigue is accumulated. When the first and second adiabatic membranes 20 and 30 are damaged due to weld defects or physical factors, it causes deterioration in adiabatic effects and leakage of LNG because vacuum states or pressurized states inside the adiabatic membranes are not maintained.
The deterioration in thermal insulation properties due to the leakage of the adiabatic membranes increases evaporation pressure or the LNG which is stored in the LNG tank. However, if the evaporation pressure becomes higher than design pressure of the LNG tank, LNG must be discharged out in order to reduce pressure of the LNG tank, and it means consumption of the stored LNG. The LNG carrier suffers great losses (the amount of damages is estimated at many billions won) if the vaporized LNG is discharged out and consumed due to excessive pressure. Additionally, if the first adiabatic membrane is damaged and leaks, it is dangerous because the vaporized LNG may be introduced into the first thermal insulator 40 and the LNG tank may be exploded. For these reasons, it is necessary to exactly measure sloshing of LNG inside the LNG tank and to design and repair the LNG tank in correspondence with the measured values.